Indoor design conditions are determined by comfort or process requirements. For comfort cooling, conditions of 75 F and 40 to 50 percent maximum relative humidity are usually recommended, although some energy codes may require higher summer temperatures.

For comfort heating, an indoor design temperature of 70 to 72 F is usually satisfactory. Many people will try to operate the systems at lower or higher temperatures than design, and this will be possible most of the time.

Most HVAC systems tend and need to be oversized for various reasons, some of which will be pointed out later. Outside design conditions are determined from published data for the specific location, based on weather bureau records.

The ASHRAE Handbook 2001 Fundamentals1 lists data for over 1000 sites in North America and throughout the world. For comfort cooling, use of the 2.5 percent values is recommended; for comfort heating, use the 99.0 percent values, except use a median of annual extremes for certain critical heating applications.

Note that the maximum wet-bulb (wb) temperature seldom occurs at the same time as the design dry-bulb (db) temperature. For sites not listed, data may be obtained by interpolation, but this should be done only by an experienced meteorologist.

The design temperature and humidity conditions should be plotted on a psychrometric chart. Then the relative humidity (RH) and enthalpy (h) can be read as well as the indoor wet-bulb temperature.